The invention relates generally to the field of radiation imaging of internal structures and, more specifically, to computerized axial tomographic (CT) X-ray scanners. Unlike conventional exposed film X-ray apparatus, the CT scanner produces narrow beams of radiation, either X-ray or gamma rays, through plural coplanar paths defining a cross-sectional or tomographic view of the patient's internal organs, such as the brain. The attenuated beams are sensed by radiation detectors whose electrical output is indicative of the intensity of the radiation received by the detector. One of the early types of CT scanners referenced in the patent literature is shown, for example, in Hounsfield U.S. Pat. No. 3,778,614. This system is generally referred to in the art as the "translate and rotate" system. A source and a single detector, for example, are aligned opposite each other on a mechanism which causes the beam path between the source and detector to move laterally across the scan circle. After rotating the source/detector carriage assembly to a new orientation, the translational scan is repeated. Readings are taken at uniformly spaced parallel beam locations and representative values are digitally stored. Data from a full set of scans involving numerous relocations of the beam path is manipulated according to known mathematics involving "back projection" to arrive at a digital representation of the tomographic image. This digital representation is converted to a tomogram which can be viewed on a cathode ray tube. Ohio-Nuclear, Inc. markets a type of translate and rotate computerized tomographic scanner under the trademark "DELTA SCAN".
The major disadvantage of the translate and rotate system is slowness of the scan mechanism due to the different alternating types of motion. The major advantages of the translate and rotate system are due to the fact that a single detector scans across the entire scan circle thus enabling sampling at any time and avoiding the need to have matched detectors or gain matching.
Another type of scan technique called "purely rotational" employs a fan beam source with a subtended detector array in a fixed relationship such that the fan beam and detector array rotate with each other. This system has a major disadvantage. Numerous detectors are required and none scans across the entire patient. Thus, the sampling resolution is lowered and gain matching of the detectors is required. The major advantage of the purely rotational system is its high scanning speed. The high speed of the scanning motion is desirable to avoid the effect on the image of the resultant displacement of organs due to a patient's breathing.
It has been found that computer image reconstruction can be accomplished with yet another arrangement of source and detectors. In this new system, the detector array is a stationary arc of uniformly spaced detectors about the center point in the scan circle. The fan pattern source revolves about the center point inside the detector array irradiating the scan circle and subtending at any given time only a fraction of the detectors in the total array. If desired, the array may be a complete circle or ring. The reconstruction algorithms are described in Lakshminarayanan, "Reconstruction from Divergent Ray Data", Technical Report No. 92, State University of New York at Buffalo, Computer Sciences Department, January, 1975.
The new type of scanning system, although requiring numerous detectors and somewhat more elaborate digital processing for reconstructing an image, provides the advantage of high scanning speed due to the single mechanical motion for rotation while also providing the capability of achieving high sampling resolution and avoiding gain matching requirements because each detector views the source across the entire scan circle.
If the circular array of detectors does not fully encircle the patient, it is possible for the patient to be exposed to unused radiation when the source approaches the terminus of its orbit and part of the fan pattern falls outside the detector array. Another problem is presented when the detectors are spaced apart throughout the array since the fan pattern is not aligned with specific detectors but instead floods the scan circle. In this case, a portion of the radiation falls between adjacent detectors and is not used for data collection. This radiation dosage is received by the patient, however, even though it is not used.
In the above three types of scanning systems, it has been found that reconstruction is more easily and accurately accomplished if the distribution of the amount of the radiation traversing the scan circle is generally a bell-shaped distribution with its peak corresponding to radiation traversing the center of the scan circle and its edges corresponding to radiation traversing the scan circle nearly tangential to the scan circle. To modify the amount of radiation attenuating filters, such as blocks of aluminum which are machined-thin near the center and thick near the edges, are inserted between the source of radiation and the detectors. See U.S. Pat. No. 3,937,963. Other shaped attenuation filters which better match the radiation distribution with the anticipated amount of attenuation along various paths through the scan circle are shown in U.S. Pat. No. 3,755,672.